Image intensifier camera systems based on television cameras or CCD cameras, storage film systems with an integrated or external readout unit, systems with optical coupling of a converter film to CCD cameras or CMOS chips, selenium-based detectors with electrostatic readout, and x-ray detectors having active readout matrices with direct or indirect conversion of the x-radiation are examples of what is known in digital x-ray imaging.
Known in x-ray imaging for the purpose of recording digital x-ray images of an object are so called solid state detectors in the case of which an x-radiation is converted directly or indirectly into electric charge, and the electric charge is stored in so called active matrices that are assembled from a multiplicity of pixel readout units. The information is subsequently readout electronically and further processed for the purpose of image generation.
In the case of direct conversion, when impinging on a direct converter layer, for example made from amorphous selenium, an x-ray quantum of the x-radiation produces a high energy electron that then in turn generates charge carriers on its path through a direct converter layer. The charge carriers are transported with the aid of an electric field to an electrode contained in the pixel readout unit and stored there as charge.
In the case of indirect conversion, when impinging on a scintillator layer an x-ray quantum of the x-radiation produces a high energy electron that then in turn generates light on its path through the scintillator layer. The light is converted into electric charge and likewise stored on photodiodes arranged below the scintillator layer and contained in the pixel readout unit.
The corresponding charge pulse, which depends chiefly on the energy of the primary x-ray quantum, is subsequently readout by the active switching elements likewise contained in the pixel readout units. Such solid state detectors are known, for example, from the article entitled “Flachbilddetektoren in der Röntgendiagnostik” [“Flat image detectors in x-ray diagnostics”] by M. Spahn, V. Heer, R. Freytag, published in the journal Radiologe 43, 2004, pages 340 to 350.
A distinction is made between a counting and an integrating x-ray detector. In a counting x-ray detector, a charge pulse is evaluated in a pixel readout unit as signal of an x-ray quantum, whereas in an integrating x-ray detector integration is carried out over all the charge pulses in a pixel readout unit.
In order to be able to distinguish background noise from a charge pulse originating from an x-ray quantum actually present in the case of a counting x-ray detector, a lower threshold value is defined in general above which a charge pulse is interpreted as a signal of an x-ray quantum. It is also possible to define an upper threshold value above which a charge pulse is defined as a signal of two or more x-ray quanta. In some counting x-ray detectors, the signal level of the charge pulse is recorded in order to be able as well to reduce the quantum energy of the respective x-ray quantum, since signal level and quantum energy are usually proportional to one another.
Counting x-ray detectors are disclosed, for example, in DE 10212638 A1 and DE 10357187 A1. The advantage of counting detectors is that it is possible to reduce the noise, while the signal-to-noise ratio can be improved.